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Claims that cannabis use is associated with lower cognitive functioning are largely based upon the findings of a single longitudinal study. The paper, published by Madeline Meier and a team of Duke University researchers in 2012, reported that the onset of cannabis use in early adolescence was associated with an average decline of eight IQ points by middle-age.

However, a critique of Meier’s study, published in the Proceedings of the National Academy of Sciences,
opined that the reported differences in IQ were consistent with
socioeconomic differences among the study’s participants and likely were
not attributable to marijuana use. (This criticism is hardly surprising
as data has historically shown
that those of greater economic means tend to test higher on IQ tests
than those who are poorer, and critics have also raised questions as to
whether the test itself may possess inherent racial biases.) It countered
that the Duke team likely “overestimate[d]” the impact of marijuana on
IQ and opined that the “true effect [of cannabis exposure] could be
zero.”

This criticism is given additional merit by the fact that several
later and better controlled studies have failed to replicate Meier’s
initial findings. For example, a British study of more than 2,000 teens reported
that cannabis exposure prior to the age of 15 “did not predict either
lower teenage IQ scores or poorer educational performance … once
adjustment is made for potential confounds.”

Researchers at the University
of Southern California, Los Angeles, and at the University of Minnesota
similarly assessed the potential relationship between cannabis and IQ in
two longitudinal investigations of adolescent twins. They reported:
“We find little evidence to suggest that adolescent marijuana use has a
direct effect on intellectual decline…. The lack of a dose–response
relationship, and an absence of meaningful differences between
discordant siblings lead us to conclude that the deficits observed in
marijuana users are attributable to confounding factors that influence
both substance initiation and IQ rather than a neurotoxic effect of
marijuana.”

In fact, even Meier appears to concede this point in her later work. Writing in the journal Addiction in 2018, she and her colleagues acknowledged:
“Short-term cannabis use in adolescence does not appear to cause IQ
decline or impair executive functions, even when cannabis use reaches
the level of dependence. Family background factors explain why
adolescent cannabis users perform worse on IQ and executive function
tests.”

Most recently, a new study published in The Lancet being widely publicized this week claims
that an estimated 30 to 50 percent of psychosis cases in Europe are due
to cannabis exposure. However, this study does not establish causation,
nor could it because of the observational nature of its design. In
fact, the authors clearly state in the abstract that they are “assuming
causality.” Given the fact that such a cause-and-effect relationship is
yet unproven and there is no consensus among experts that such
causation exists, this would appear to be a troubling leap for the
authors to take and it should not go unnoticed. Moreover, it is well
established that those with psychiatric illness typically use all
intoxicants at greater rates than the general public, so the fact
that those admitted to institutions for first-episode psychosis are
more likely to consume cannabis than are those in the general population
is hardly surprising. But it is not evidence that marijuana in any way
causes the condition. Finally, the fact that cannabis has been
used by various populations for decades at disparate rates, yet rates of
psychosis and other psychiatric disorders have generally remained static over this same period of time, strongly argues against a direct causal relationship.

The Developing Brain

While the use of alcohol has been unequivocally associated with
deleterious effects on brain morphology in both adults and adolescents,
claims that cannabis exposure significantly damages the developing brain
are far less persuasive.

Specifically, a team of University of Colorado investigators in 2017
assessed the impact of marijuana exposure and alcohol ingestion on brain
structure in groups of adults and adolescents. They reported
that, “Alcohol use severity is associated with widespread lower grey
matter volume and white matter integrity in adults, and with lower grey
matter volume in adolescents.” (Changes in white matter are associated
with the onset of certain neurodegenerative diseases while grey matter
contains most of the brain’s neurons.) By contrast, “no associations
were observed between structural measures and past 30-day cannabis use
in adults or adolescents.”

In 2018, researchers from the University of Pennsylvania, Perlman
School of Medicine performed a systematic review of 69 studies
(published between the years 1973 and 2017) assessing the relationship
between adolescent cannabis use and cognition. Researchers reported no
significant long-term deficits in memory, attention or other aspects of
cognitive functioning that could be independently attributed to cannabis
use, regardless of subjects’ age of initiation.

They concluded
that, “Associations between cannabis use and cognitive functioning in
cross-sectional studies of adolescents and young adults are small and
may be of questionable clinical importance for most individuals….
[R]esults indicate that previous studies of cannabis in youth may have
overstated the magnitude and persistence of cognitive deficits
associated with [marijuana] use.”

Most recently, University of Pennsylvania researchers compared brain
scans of adolescents who used cannabis occasionally (one to two times
per week) and frequently (more than three times per week) versus matched
controls (similarly aged teens who abstained from the plant). They concluded,
“There were no significant differences by cannabis group in global or
regional brain volumes, cortical thickness, or gray matter density, and
no significant group by age interactions were found…. In sum, structural
brain metrics were largely similar among adolescent and young adult
cannabis users and non-users.”

Long-Term Exposure

While some have theorized that marijuana exposure over the long term
may be associated with an adverse impact on the brain, clinical
assessments of frequent cannabis consumers challenge concerns. For
instance, cumulative lifetime cannabis exposure is not associated with deficits in cognitive performance following drug abstinence.

Long-term exposure is also not associated with changes in working memory. In a 2015 Canadian clinical trial
assessing the safety of daily cannabis consumption, users demonstrated
no identifiable differences in neurocognitive skills compared to
non-using controls. These findings were similar to those of a U.S. trial
assessing the health of four Compassionate IND patients (federally authorized medical marijuana consumers) who had smoked cannabis daily for a period of up to 27 years. Authors concluded that, “No consistent or attributable neuropsychological or neurological deterioration has been observed.”

Most recently, University of Colorado scientists compared magnetic
resonance imaging scans in 28 cannabis users over the age of 60 versus
matched controls. Cannabis-consuming participants in the study, on
average, had used marijuana weekly for 24 years. The authors reported
that long-term cannabis exposure “does not have a widespread impact on
overall cortical volumes while controlling for age” despite over two
decades of regular cannabis use on average. This is in contrast to the
large, widespread effects of alcohol on cortical volumes that might be
expected to negatively impact cognitive performance.

Facts, Not Fears, Should Guide Marijuana Policy

An estimated one out of five Americans now resides in a jurisdiction
where the adult use of cannabis is legally regulated, while more than 30
states have enacted statutes authorizing its medical access. Public
support for marijuana policy reform has also steadily increased over the
past decade, and now two out of three Americans endorse
cannabis legalization. It is time for politicians to reject the
unsubstantiated “reefer rhetoric” and fear-mongering of the past and
move forward to amend U.S. cannabis laws in response to the emerging
science, public opinion and the plant’s rapidly changing cultural
status.

Editor’s note: This article was updated on March
20, 2019, to address a study that estimates 30-50 percent of psychosis
cases in Europe are due to cannabis exposure.